Screw compressors have been conventionally widely used for applications of compressing a refrigerant or air. As described in Japanese Patent Publication No. 2004-137934, for example, a screw compressor capable of changing its compression ratio using a slide valve is also known.
Specifically, Japanese Patent Publication No. 2004-137934 shows a single-screw compressor including one screw rotor. This single-screw compressor includes a slide valve movable along the axis of the screw rotor. This slide valve has a discharge port. In the single-screw compressor, rotation of the screw rotor causes a fluid to be sucked in a compression chamber formed by a helical groove on the screw rotor and compressed. When the compression chamber comes to communicate with the discharge port of the slide valve, the compressed fluid is discharged from the compression chamber through the discharge port.
In the single-screw compressor of Japanese Patent Publication No. 2004-137934, when the slide valve moves, the discharge port thereof also moves. A change in the position of the discharge port changes the volume of the compression chamber at the time when the compression chamber starts communicating with the discharge port. Thus, movement of the slide valve changes the compression ratio accordingly.
A configuration of a conventional single-screw compressor will be described with reference to FIGS. 23-25.
As illustrated in FIG. 23, in the single-screw compressor, a screw rotor (520) is inserted in a cylinder part (511) in a casing (510). The screw rotor (520) is coupled to an electric motor, not shown, through a driving shaft (525). Helical groove (521) on the screw rotor (520) form a compression chamber. The helical grooves (521) are engaged with gates of a gate rotor. Rotation of the screw rotor (520) causes a fluid to be sucked from a low-pressure space (515) into the compression chamber, and compressed.
A slide valve (530) is disposed at the side of the screw rotor (520). As illustrated in FIG. 25, the slide valve (530) includes a valve body (531), a guide (534), and a joint (535). The valve body (531) has a columnar shape. The front surface (532) of the valve body (531) is curved and faces the outer periphery of the screw rotor (520). The back surface (533) of the valve body (531) is a cylindrical surface having a radius of curvature smaller than that of the front surface (532). The front surface of the guide (534) is in slidable contact with the outer peripheral surface of the bearing holder (512) fixed to the casing (510). The joint (535) is formed in a rod shape, and couples the valve body (531) and the guide (534) together. In the slide valve (530), space between the valve body (531) and the guide (534) serves as a discharge port (536). The fluid compressed in the compression chamber is discharged to a high-pressure space (516) through the discharge port (536).
A sealing projection (513) is formed on a portion of the casing (510) facing the back surface (533) of the valve body (531). A projection end surface (514) of the sealing projection (513) is curved to have a radius of curvature substantially equal to that of the back surface (533) of the valve body, and is in slidable contact with the back surface (533) of the valve body (531). The projection end surface (514) of the sealing projection (513) separates the low-pressure space (515) and the high-pressure space (516) from each other when the front end surface (514) is in slidable contact with the back surface (533) of the valve body (531).